1. Field of the Invention
The present invention is directed to a power amplifier of the type having two output stages connected in series at the output side of the power amplifier, driven by a setting valve, and method for the generation of control signals for such a power amplifier. The inventive power amplifier and method can be used for all power amplifier applications, but are particularly suited for use in a medical apparatus, particularly a magnetic resonance tomography apparatus. In this context, the inventive power amplifier can be utilized as a gradient amplifier in order to supply a gradient coil with current.
2. Description of the Prior Art
Magnetic field gradients are generated by gradient coils during the operation of a magnetic resonance tomography apparatus or a magnetic resonance system. Each gradient coil has current flowing therein that, in an exactly defined shape of the current curve, can reach values up to, for example, 300 A or more. The shape of the current curve often exhibits steep edges. In order to achieve the high rates of current change that are required, it can be necessary to apply a voltage of, for example, 2000 V or more to the gradient coil. Such power demands can be met only with great outlay, if at all, with a single output stage.
U.S. Pat. No. 5,546,299 discloses a power amplifier having at least two controllable voltage sources connected in series at its output side, in a power supply for a gradient amplifier. A number of control units are fashioned as modulators for generating pulse-width drive signals for the voltage sources. In an alternative embodiment described in German 43 04 517, a number of voltage sources are driven with switching signals offset in time for the peak load mode.
German OS 40 17 207 discloses means for feeding a dipole load wherein a number of DC voltage step-down regulators are driven according to the principle of pulse-width modulation. The uniformly large clock cycles are respectively offset by 1/n of the period duration.
The dissertation "Der Schwingkreiswechselrichter mit Gegenspannungslast zur Versorgung von Verbrauchern mit gepulstem Gleichstrom" by Werner Kuhnel, Technical Faculty of the University of Erlangen, 1986, pages 133-134, discloses a multi-DC-AC converter for high-power x-ray generators. An offset drive of the individual DC-AC converters is optionally provided.
United States Application having Atty. Docket No. P99,0811 ("Power Amplifier and Method for the Drive of a Power Amplifier," Schweighofer) filed simultaneously herewith and assigned to the same assignee (Siemens AG) as the present application, discloses driving a number of output stages, connected in parallel at the output side, with offset switching clocks in a gradient amplifier in order to reduce the ripple of the output voltage.
Insofar as details of the output stage drive are disclosed at all in the above publicly available documents, complicated modulation circuits are utilized. Moreover, these circuits are often not optimally designed, for example, in view of the system dynamics or the effective switching frequency or of the output stage load of the power amplifier.